1. Field of the Invention
The present invention relates to a circuit for adjustment of the optical black level of a solid state image-pickup device and an apparatus using the same.
2. Description of the Related Art
An example of an optical black level compensating circuit (OB compensating circuit or optical black compensating circuit) of a CCD, CMOS sensor, or other image sensor is shown in FIG. 5. An optical black level compensating circuit is a circuit for correcting the black level of a black level signal output region of the image sensor so as to match with the optical black code. The optical black level compensating circuit of FIG. 5 shows part of a signal processing circuit after a signal is output from the image sensor etc. This is provided with a low pass filter (LPF) 2 including a sample/hold circuit (S/H) 81, an analog-digital converter (ADC) 82, a subtractor 83, a decoder 84, a current-output type digital-to-analog converter (I-DAC) 85, external capacitors 87 and 88, and an amplifier 86.
A signal SIN output from an image sensor (not shown) is supplied to the input terminal T1. This input terminal T1 is connected to the input of the sample/hold circuit 81, while the output of the sample/hold circuit 81 is connected to the next analog-to-digital converter 82.
The output of the analog-to-digital converter 82 is connected to an output terminal T2 of the OB compensating circuit and one input terminal of the subtractor 83. The other input terminal of the subtractor 83 is connected to the output of an optical black code circuit and supplied with a digitalized optical black level correction signal. Further, the output of the subtractor 82 is connected to the input of the decoder 84, while the output of the decoder 84 is connected to the input of the current-output type digital-to-analog converter 85. The current-output type digital-to-analog converter 85 converts the input digital signal to an analog signal. The output is connected to the input of the amplifier 86 and the external capacitor 88 forming parts of the low pass filter 2. The output of this amplifier 86 is connected to the external capacitor 87 and the other input of the sample/hold circuit 81.
Here, the capacitors forming part of this low pass filter 2 have to be set to large capacities for correcting the optical black level. Since the capacities are large 0.1 μF or so, formation in an IC is difficult. The capacitors are therefore connected to the outside the IC as the external capacitors 87 and 88.
The analog video signal SIN output from the image sensor etc. is input to sample/hold circuit 81 where it is sampled at a predetermined period. The held signal is supplied to the next analog-to-digital converter 82. The supplied analog video signal is converted by the analog-to-digital converter 82 to a digital signal and, for example, becomes a 12- to 16-bit digital signal which is supplied from the output terminal to the next digital signal processing circuit. This digital signal is supplied to the subtractor 83 where subtraction is performed using this digital signal and the digital optical black code (optical black level correction code) supplied from the input terminal T3. The error signal obtained as a result (digital signal) is output to the decoder 84.
A code for selecting the number of bits is generated based on the data supplied to the decoder (DEC) 84. This generated code is used to control the current-output type digital-to-analog converter 85. As a result, the digitalized error signal from the current-output type digital-to-analog converter 85 is output as an analog current (signal). Next, this analog current (signal) is supplied to the low pass filter 2 and converted to voltage. At this time, the charging/discharging times of the capacitors are determined by the current supply capacity of the constant current circuit of the current-output type digital-to-analog converter 85. The voltage signals appearing at the capacitors are supplied to the other input of the above sample/hold circuit 81. The sample/hold circuit 81 adds or subtracts these with the input signal to correct the output signal of the sample/hold circuit 81. However, when the charging/discharging times of the capacitors are long, the capacitor voltages will not reach the target values in the sampling clock period and an error voltage will be generated. From this, the current supply capability of the current-output type digital-to-analog converter 85 and the capacities of the capacitors must be set so as not to generate error voltage. The input analog video signal is corrected by the OB code of the correction value of the black level.
However, since the above-mentioned optical black level compensating circuit has the low pass filter 2 provided with the external capacitors 87 and 88, there was the problem of the correction value of the black level ending up deviating due to fluctuations in the electrostatic capacity caused by changes in humidity, leakages due to humidity, leakages due to deterioration of properties along with aging, etc.